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<b><a class="bz_bug_link
bz_status_NEW "
title="NEW - JSC: DFG::SpeculativeJIT::compile special case for MIPS for PutByValWithThis"
href="https://bugs.webkit.org/show_bug.cgi?id=157741#c9">Comment # 9</a>
on <a class="bz_bug_link
bz_status_NEW "
title="NEW - JSC: DFG::SpeculativeJIT::compile special case for MIPS for PutByValWithThis"
href="https://bugs.webkit.org/show_bug.cgi?id=157741">bug 157741</a>
from <span class="vcard"><a class="email" href="mailto:guijemont@igalia.com" title="Guillaume Emont <guijemont@igalia.com>"> <span class="fn">Guillaume Emont</span></a>
</span></b>
<pre>Comment on <span class=""><a href="attachment.cgi?id=279023&action=diff" name="attach_279023" title="Patch">attachment 279023</a> <a href="attachment.cgi?id=279023&action=edit" title="Patch">[details]</a></span>
Patch
View in context: <a href="https://bugs.webkit.org/attachment.cgi?id=279023&action=review">https://bugs.webkit.org/attachment.cgi?id=279023&action=review</a>
<span class="quote">>>>> Source/JavaScriptCore/dfg/DFGSpeculativeJIT32_64.cpp:2947
>>>> + m_jit.move(baseTag, GPRInfo::argumentGPR3);
>>>
>>> This is wrong if basePayload/baseTag are aliased to argument registers w/ each other.
>>> i.e, if basePayload is argumentGPR3 and baseTag is argumentGPR2, this code will do the wrong thing.
>>
>> I thought this was not possible because the argumentGPR's are not in GPRInfo::toRegister() (and not counted in GPRInfo::numberOfRegisters). Including them might be an idea for the future though, as I suspect more registers available might improve performances (and ARM does that so it's probably possible), but I think that's a trickier change, and I'd like to get the build unbroken.
>
> I don't quite understand your response here. Tag/Payload regs will be allocated by the DFG's register allocator. It will happily use argument registers.
> I don't think this makes the code harder to read. All code we write that sets up a call frame must account for this.</span >
Sorry if that wasn't clear, and there is a possibility that I misunderstand something. What I mean is that DFG::RegisterBank<GPRInfo>::allocate() (see ::allocateInternal()) uses GPRInfo::toRegister() (and GPRInfo::numberOfRegisters as RegisterBank::NUM_REGS) as its source of registers. In the case of MIPS, GPRInfo::toRegister() never returns an argumentGPRx. Therefore baseTag and basePayload cannot be an argumentGPRx register. That's my current understanding at least.</pre>
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